Re: [PATCH] genirq: Keep handle_nested_irq() from touching desc->threads_active

[Thomas Gleixner <tglx@linutronix.de>]

On Mon, Jun 10 2024 at 02:30, Peng Liu wrote:
> handle_nested_irq() is supposed to be running inside the parent thread
> handler context. It per se has no dedicated kernel thread, thus shouldn't
> touch desc->threads_active. The parent kernel thread has already taken
> care of this.

No it has not. The parent thread has marked itself in the parent threads
interrupt descriptor.

How does that help synchronizing the nested interrupt, which has a
separate interrupt descriptor?

> Fixes: e2c12739ccf7 ("genirq: Prevent nested thread vs synchronize_hardirq() deadlock")
> Cc: stable@vger.kernel.org

There is nothing to fix.

> Signed-off-by: Peng Liu <iwtbavbm@gmail.com>
> ---
>
> Despite of its correctness, I'm afraid the testing on my only PC can't
> cover the affected code path. So the patch may be totally -UNTESTED-.

Which correctness?

The change log of the commit you want to "fix" says:

    Remove the incorrect usage in the nested threaded interrupt case and
    instead re-use the threads_active / wait_for_threads mechanism to
    wait for nested threaded interrupts to complete.

It's very clearly spelled out, no?

Thanks,

        tglx